1. Field of the Invention
This invention relates to electronic circuits supplied with a voltage Vcc in which a voltage greater than Vcc must be generated.
2. Discussion of the Related Art
A typical example is that of integrated circuits comprising non-volatile memories with floating-gate transistors; programming such memories requires a programming voltage Vpp much greater than the normal supply voltage Vcc. In order to allow the user to use this memory with only one supply voltage Vcc, measures are taken for the integrated circuit to have an internal means to generate the voltage Vpp from Vcc. Typically, Vcc has a value of 5 volts or even less and Vpp has a value of 15 volts or more. The future trend is toward a voltage Vcc lower than 2 volts, while Vpp can be above 15 volts.
The circuit for the generation of Vpp is a voltage booster circuit whose principle is that of the "charge pump".
The block diagram of the charge pump is shown in FIG. 1. The charge pump comprises a series of diode and capacitor stages and switches providing for the switching of the capacitor connections between Vcc and ground and depending on two non-overlapping periodic phases Phi1, Phi2. Each stage includes two capacitors C1 and C2 and two diodes D1 and D2. In the first phase of Phi1, the first capacitor C1 is charged to the supply voltage Vcc. In the second phase of Phi2, it is partly discharged into the second capacitor C2. Then C1 is charged (Phi1) again. The diodes prevent the second capacitor C2 from discharging, so that its load progressively increases until it reaches a value which exceeds Vcc (up to a theoretical maximum of 2Vcc if the voltage drops in the diodes are not taken into account).
In-order to obtain a higher voltage, n successive stages are cascade-connected. The obtained voltage amounts to (n+1)Vcc or, more precisely, if the threshold voltage Vd of the diodes is taken into account, to (n+1) (Vcc-Vd).
In order to obtain a sufficient output voltage value with the smallest possible number n of stages, it has already been suggested to replace the diodes D1 and D2 with transistors, across which no significant voltage drop is created because their resistance is negligible when they are in conductive mode. The block diagram in FIG. 2 shows this. Since the transistors have a threshold voltage Vt (minimum gate-to-source voltage below which they are not conductive), certain transistors are arranged to have their gates driven by a voltage level which is higher than (Vcc+Vt). Thus they are made conductive (i.e., no significant voltage drop across them) even if their sources and their drains are at Vcc, which eliminates the threshold voltage problem. With a charge pump comprising n stages, an output voltage up to (n+1)Vcc can be obtained, which is more favourable than the result of diode circuits. But it implies that the gates of certain transistors can be driven using a voltage slightly greater than Vcc.
For this reason, the charge pump diagram in FIG. 2 shows two drive signal pairs: Phi1 and Phi2 on the one hand as shown in FIG. 1, which switch between two voltage levels 0 and Vcc; and Phi1b and Phi2b on the other hand, synchronized with Phi1 and Phi2 respectively, but switching between voltage levels 0 and VB, where VB has a greater level than Vcc, preferably greater than or equal to Vcc+Vt. FIG. 3 shows schematically the switching phases of the charge pump according to FIG. 2.
A significant parameter of the charge pump is its "fan-out", which is the number of loads it can supply without any excessive output voltage attenuation. Computations show that the fan-out is inversely proportional to the number of stages of the pump and is proportional to the switching frequency of the pump, i.e., the frequency of signals Phi1, Phi2, Phi1b, Phi2b.
Therefore, the switching frequency must be quite stable, or at least it should decrease as little as possible when the number of stages of the pump increases. It is also desirable that the switching frequency be as stable as possible with respect to the supply voltage Vcc.
In order to generate switching drive signals Phi1, Phi2, Phi1b, and Phi2b, it has already been suggested to use comparatively complex oscillator circuits which have the disadvantage of a poor frequency stability, both with respect to the supply voltage changes and also with respect to the number of stages or, more generally, to the structure of the charge pump they actually control.
Therefore, the circuits utilized in the prior art have a charge pump frequency which depends on the number of the charge pump stages in a significant way, which makes the circuit design rather difficult and thus general-purpose diagrams, capable of being transposed from one circuit to another one, cannot be drawn for the charge pump.
It is an object of this invention to provide an improved charge pump diagram.